The source of heat driving the convection currents is radioactive
decay which
is happening deep in the Earth.

The edges of these plates, where they move against each other,
are sites of
intense geologic activity, such as earthquakes, volcanoes, and mountain
building.

Plate tectonics is a relatively new theory and it wasn't until the
1960's
that Geologists, with the help of ocean
surveys,
began to understand what goes on beneath our feet.

Where is
the Evidence for Plate Tectonics?

The continents seem
to fit
together like a giant jigsaw puzzle:

If you look at a map, Africa seems to snuggle nicely
into the
east coast of South America and the Caribbean sea. In 1912 a German
Scientist
called Alfred Wegener proposed that these two continents were once
joined together
then somehow drifted apart. He proposed that all the continents were
once
stuck together as one big land mass called Pangea. He believed that
Pangea was
intact until about 200 million years ago

CONTINENTAL DRIFT

The idea that continents can drift about is called, not surprisingly,
CONTINENTAL
DRIFT.

When Wegener first put forward the idea in 1912 people thought he was
nuts.
His big problem was that he knew the continents had drifted but he
couldn't explain
how they drifted. The old (AND VERY WRONG!!) theory before this time was
the "Contraction
theory" which suggested that the planet was once a molten ball and in
the
process of cooling the surface cracked and folded up on itself. The big
problem with this idea was that all mountain ranges should be
approximately the
same age, and this was known not to be true. Wegener's explanation was
that
as the continents moved, the leading edge of the continent would
encounter resistance
and thus compress and fold upwards forming mountains near the leading
edges of
the drifting continents. Wegener also suggested that India drifted
northward into
the Asia forming the Himalayas and of course Mount Everest.

SEA FLOOR SPREADING

It is hard to imagine that these great big solid slabs of rock could
wander
around the globe. Scientists needed a clue as to how the continents
drifted. The
discovery of the chain
of mountains that lie under
the oceans was the clue that they were waiting for.

PLATES
ARE CREATED: In
the diagram below you can see that the continental crust is beginning to
separate
creating a diverging plate boundary. When a divergence occurs
within a
continent it is called rifting. A plume
of hot magma
rises from deep within the mantle pushing up the crust and causing
pressure forcing
the continent to break and separate. Lava flows and earthquakes would be
seen.
In the diagram below you can see that the continental crust is beginning
to separate
creating a diverging plate boundary. When a divergence occurs
within a
continent it is called rifting. A plume
of hot magma
rises from deep within the mantle pushing up the crust and causing
pressure forcing
the continent to break and separate. Lava flows and earthquakes would be
seen.

This is an example of a divergent plate
boundary
(where the plates move away from each other). The Atlantic Ocean was
created by
this process. The mid-Atlantic Ridge is an area where new sea floor is
being created.

As the rift valley expands two
continental plates
have been constructed from the original one. The molten rock continues
to push
the crust apart creating new crust as it does.

As the rift valley expands, water
collects forming
a sea. The Mid-Atlantic Ridge is now 2,000 metres above the adjacent sea
floor,
which is at a depth of about 6,000 metres below sea level.

The sea floor
continues
to spread and the plates get bigger and bigger. This process can be seen
all over
the world and produces about 17 square kilometres of new plate every
year.

Now let's think back to our plates being created at the mid-ocean
ridges, it
seems to be a good idea but if this is the only type of plate movement
then the
world would get bigger and bigger. In fact the world has remained the
same size.
So if plates are being created at the mid-ocean ridges then they must be
being
consumed somewhere else in the world.

PLATES
ARE DESTROYED (SUBDUCTION):

This is a convergent plate boundary,
the plates
move towards each other. The amount of crust on the surface of the earth
remains
relatively constant. Therefore, when plates diverge (separate)
and form
new crust in one area, the plates must converge (come together)
in another
area and be destroyed. An example of this is the Nazca plate being
subducted under
the South American plate to form the Andes Mountain Chain.

Here we can see the oceanic plate moving from left to right. The top
layer
of the mantle and the crust (all called the lithosphere) sinks beneath
the continent.
A deep ocean trench is formed. Water gets carried down with the oceanic
crust
and the rocks begin to heat up as they travel slowly into the earth.
Water is
then driven off triggering the formation of pools of molten rock which
slowly
rises. The plate moves downwards at a rate of a few centimetres per
year. The
molten rock can take tens of thousands of years to then either:

Solidify slowly underground as intrusive igneous rock
such as
granite.

or

Reach the surface and erupt as lava flows. Cooling rapidly to
form
extrusive igneous rock such as basalt.

The floor of the Easter Pacific is moving towards South America at a
rate of
9 centimetres per year. It might not seem much but over the past 10
million years
the Pacific crust has been subducted under South America and has sunk
nearly 1000
kilometres into the Earth's interior.

Types of Convergent
Boundaries

The example above showed what happened when the dense oceanic plate
subducts
under a lighter continental plate (ie, oceanic - continental). Two other
types
of subduction can take place:

When two oceanic plate meet each other
(oceanic-oceanic)
this often results in the formation of an island arc system. As the
subducting
oceanic crust melts as it goes deeper into the Earth, the newly-created
magma
rises to the surface and forms volcanoes. If the activity continues, the
volcano
may grow tall enough to breech the surface of the ocean creating an
island.

The key to subduction seems to be water which acts as a kind of
lubricant as
the heavier plate slips underneath the lighter plate.

I must not forget to mention the Himalayas and
Mount Everest
because this is the third example of plate movement

Millions of years
ago India
and an ancient ocean called the Tethys Ocean were sat on a tectonic
plate. This
plate was moving northwards towards Asia at a rate of 10 centimetres per
year.
The Tethys oceanic crust was being subducted under the Asian Continent.
The ocean
got progressively smaller until about 55 milion years ago when India
'hit' Asia.
There was no more ocean left to lubricate the subduction and so the plates
welled up to form the High Plateau of Tibet and the Himalayan Mountains.
The
continental crust under Tibet is over 70 kilometres thick. North of
Katmandu,
the capital of Nepal, is a deep gorge in the Himalayas. the rock here is
made
of schist and granite with contorted and folded layers of marine
sediments which
were deposited by the Tethys ocean over 60 million years ago.

The fourth type of plate movement involves plates
sliding
past one another without the construction or destruction of crust. This
boundary
is called a conservation zone because plate is neither created nor
destroyed An
example of such a boundary is the San
Andreas fault in California. The force needed to move billions of
tonnes of
rock is unimaginable. When plates move some of the energy is released as
earthquakes.